A variable camshaft timing (VCT) system enables adjustment of an angular position of a camshaft to vary valve timing of an engine in order to accommodate various operating conditions of the engine. For example, during low load conditions the VCT system may change the valve timing via camshaft position adjustment to increase operating efficiency and increase fuel economy. Correspondingly, during high load conditions the VCT system may change the valve timing via camshaft position adjustment to increase engine output.
In one example, a VCT system adjusts an angular position of the camshaft relative to a reference crankshaft position measurement by controlling an electric motor that is operable to vary the angular position of the camshaft. In a typical feedback control approach, an error in angular position of the camshaft is observed before the VCT system intervenes to operate the electric motor to correct for the angular error of the camshaft position.
However, the inventors have recognized several potential issues with such a feedback control system. As an example, an update rate of a camshaft position measurement is not fast enough to compensate for an angular position error observed by feedback control. As such, camshaft position adjustment to compensate for the angular position error is delayed causing a reduction in control accuracy. On the other hand, the update rate of the camshaft position measurement can be increased by installing a camshaft position sensor that includes a target having more teeth, but such a position sensor would increase the production cost of the system.
In one example, the above mentioned issues may be addressed by a method for controlling an electric motor for positioning an adjustment mechanism to vary cylinder valve operation. The method may comprise adjusting a torque output of the electric motor to counteract an anticipated periodic torque disturbance applied to the adjustment mechanism from a valvetrain source to position the adjustment mechanism at a desired position.
As an example, the adjustment mechanism may include a variable camshaft timing actuator that adjusts an angular position of a camshaft to vary cylinder valve timing. During operation, torque disturbances that occur periodically, for example, based on rotation of the camshaft can cause an angular position error of the camshaft. However, since the torque disturbances occur on a periodic basis, they can be anticipated and characterized. In one particular example, the periodic disturbance torques are used to generate a feedforward control signal to adjust the output torque of the electric motor. By adjusting the output torque of the electric motor based on the anticipated torque disturbances, camshaft angular position error can be substantially reduced or eliminated. In other words, torque can be provided by the electric motor to counteract the torque disturbances as they occur to maintain the camshaft at a desired angular position. On the other hand, in a feedback response system, a camshaft angular position error occurs before corrective adjustment is provided.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.